The present invention relates in general to field emission devices and in particular, to cold cathodes used in field emission devices.
Carbon flake is a carbon material with a graphitic structure. It can be as thin as one or more layers of sp2-bonded carbon atoms (graphite layers), and can be very long in two other dimensions. The length of a flake can be on the order of microns, whereas the thickness is on the order of nanometer or tens of nanometers. Thus, the aspect ratio for this material is very high. A flake, by its nature, is a system of ordered or turbostratic graphite layers. Carbon flakes fall into a class of nanostructured carbon materials. The flakes can be grown by several methods that fall into the following categories:
1. DC Glow Discharge. This method involves a direct current glow discharge between two electrodes in a gas environment. The plasma between the two electrodes is of the order of 1000xc2x0 C. or higher. This method produces carbon flakes along with other types of carbon materials such as carbon nanotubes. This method is used for depositing directly onto a substrate.
2. Thermal CVD (Chemical Vapor Deposition) Method. In this method, a carbon precursor gas and a substrate are heated to a temperature of 600xc2x0 C. and higher while thermal decomposition of the precursor is observed. The substrate has a catalyst on the working surface, which gives rise to growing carbon structures like carbon nanotubes and carbon flakes. A bias voltage can be used to make carbon nanostructures grow straight. This method is used for depositing directly onto a substrate.
Carbon nanostructures are believed to be good field emitters due to their chemical stability, sufficient electric conductivity, and low value of electron affinity. Carbon nanostructures fall into two categories with respect to their phase composition and morphology, namely, carbon films with low electron affinity, and those with a high aspect ratio:
1. Diamond films, diamond-like films, and nanocrystalline diamond fall into the first category where the low or negative values of electron affinity are essential, as described in F. J. Himspel, J. A. Knapp, et al. Phys. Rev. B, vol. 20, p. 624, 1979.
2. Carbon nanotubes and filaments fall into the second category where high values of electric field enhancement factors are essential.
These two groups of carbon materials specify two different approaches to making low-field carbon cold cathodes. Cold cathode materials from the second category are proven to have better field emission performance than those materials from the first category. The general problem with these materials is that the field emission properties of these carbon nanostructures are not always good.
The films from the first category have a rather poor defect-induced conductivity and low field enhancement factors, and, hence, lower density of emission sites and higher extraction fields. Along with susceptibility to ion bombardment, these films are sensitive to a chemical nature of bombarding species since the adsorbed atoms can significantly increase the electron affinity of these materials.
The films from the second category are also susceptible to degradation of the sharp tips of the carbon fibers and nanotubes. The increase of ultimate emission current requires higher density of nanotubes. However, too densely packaged nanotubes or nanofibers need higher extraction fields due to the screening effect that lowers the local electric field near the tips. Electrostatic calculations show that the optimal distance between the nanotubes should be on the order of the nanotube average height (See L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J. M. Bonard, K. Kern, Applied Physics Letters, vol. 76, p. 2071, 10, Apr. 2000). These materials have higher electron affinity due to the graphitic nature of the carbon that they are formed of.
The problems with these materials lead to arcing and poor operation of field emission devices. One wishes to obtain good-performance cold cathodes that are robust, considerably less susceptible to ion bombardment, have high aspect ratio, and chemically inert.
The present invention addresses the foregoing needs by providing a carbon flake field emitter that is grown by a plasma-assisted chemical vapor deposition method on a semiconductor or metal substrate. A mixture of hydrogen (H2) and methane (CH4) (or other hydrocarbon gasses or liquids) is used as a carbon precursor. The substrate is heated to a high temperature above 600 C so that decomposition of carbon containing gas species occurs near the surface of the substrate.
The average distance between the emitting edges of the flakes is on the order of their size, which provides optimal field emission conditions.
Carbon flakes preferably originate from an interfacial layer of carbon material initially formed on the substrate at the beginning of the deposition. Since it is a carbon-to-carbon chemical bonding, the flakes have good adhesion to this layer.
Carbon flake field emitters differ from the other carbon-based cold cathodes in the following:
1. Better electric conductivity since the carbon flakes have a graphitic structure. As well as in graphite, the conductivity along the basal plane of the flake is almost metallic.
2. More robust than nanofibers since they are a two-dimensional structure rather than one-dimensional like the materials from the second category.
3. An increasingly extended emission area over the flake edge that enables higher emission currents.
4. Less susceptible to spoiling the field enhancement factor since this factor is a square root function of a length-to-thickness ratio for edge emission unlike a linear function of this ratio for tip emission from carbon fibers and nanotubes.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.